1. Field of the Invention
The present invention relates to a nitride semiconductor light-emitting device, and more particularly, it relates to a nitride semiconductor light-emitting device improvable in light extraction efficiency.
2. Description of the Background Art
In the structure of a conventional nitride semiconductor light-emitting diode, serving as a nitride semiconductor light-emitting device, having a p-type nitride semiconductor layer forming a light extraction side, an electrode formed on the surface of the p-type nitride semiconductor layer must satisfy the following three conditions:
First, the electrode must have high transmittance with respect to light emitted from an emission layer of the nitride semiconductor light-emitting diode.
Second, the electrode must have sufficient resistance and a sufficient thickness to be capable of sufficiently diffusing an injected current in the plane of the emission layer.
Third, the electrode must have low contact resistance with respect to the p-type nitride semiconductor layer.
In order to satisfy the aforementioned first to third conditions, a transparent conductive film of ITO (indium tin oxide) has recently been used as an electrode provided on the surface of a p-type nitride semiconductor layer in a light extraction direction of a nitride semiconductor light-emitting diode.
In the transparent conductive film of ITO, however, photochemical properties irreversibly change under a high temperature to lower transmittance for visible light, leading to limitation of a temperature region for a process after formation of the transparent conductive film. Further, the transparent conductive film is disadvantageously deteriorated and blackened when driven with high current density.
In order to solve the aforementioned problems, Japanese Patent Laying-Open No. 08-097468, for example, discloses a method of attaining excellent ohmic contact by employing InGaN having smaller band gap energy than GaN for a p-type nitride semiconductor layer of a group III nitride semiconductor light-emitting device provided with a light extraction electrode in place of p-type GaN thereby lowering the barrier between the p-type nitride semiconductor layer and the electrode formed on the surface thereof.
Japanese Patent Laying-Open No. 11-340509 discloses a method of reducing a driving voltage by forming a p-type nitride semiconductor layer provided with an electrode in a superlattice structure obtained by alternately stacking first and second nitride semiconductor layers having different compositions and introducing In into at least the first nitride semiconductor layer in the aforementioned first and second nitride semiconductor layers.
In each of the methods disclosed in Japanese Patent Laying-Open Nos. 08-097468 and 11-340509, however, it is difficult to attain excellent ohmic contact by lowering the barrier between the p-type nitride semiconductor layer and the electrode formed on the surface thereof.
Therefore, each of Japanese Patent Laying-Open Nos. 2005-229085, 2005-268601, 2005-268739 and 2006-135311 discloses a method of forming an n-type nitride semiconductor layer constituting a tunnel junction with a p-type nitride semiconductor layer on the p-type nitride semiconductor layer and forming an electrode on the n-type nitride semiconductor layer.
According to this method, contact resistance can be reduced, a driving voltage can be lowered and high-output driving can be implemented as compared with the structure obtained by forming the electrode on the surface of the p-type nitride semiconductor layer.
In the method disclosed in each of the aforementioned Japanese Patent Laying-Open Nos. 2005-229085, 2005-268601, 2005-268739 and 2006-135311, however, light extraction efficiency is still insufficient. Therefore, further improvement of the light extraction efficiency is awaited.